Accepted Manuscript A model biorefinery for avocado (Persea americana mill.) processing Javier A. Dávila, Moshe Rosenberg, Eulogio Castro, Carlos A. Cardona PII: DOI: Reference:
S0960-8524(17)30964-1 http://dx.doi.org/10.1016/j.biortech.2017.06.063 BITE 18300
To appear in:
Received Date: Revised Date: Accepted Date:
20 April 2017 9 June 2017 10 June 2017
Please cite this article as: Dávila, J.A., Rosenberg, M., Castro, E., Cardona, C.A., A model biorefinery for avocado (Persea americana mill.) processing, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech. 2017.06.063
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A model biorefinery for avocado (Persea americana mill.) processing
Javier A. Dávila a*; Moshe Rosenberg b; Eulogio Castro c; Carlos A. Cardona d1
Chemical Engineering Program, Department of Engineering. Universidad Jorge Tadeo Lozano, 110311,
Bogotá Colombia. b
Department of Food Science and Technology, University of California, Davis, Davis, CA 95616, USA.
Departamento de Ingeniería Química, Ambiental y de los Materiales, Universidad de Jaén, Campus las
Lagunillas, Spain. d
Instituto de Biotecnología y Agroindustria, Departamento de Ingeniería Química. Universidad Nacional
de Colombia sede Manizales. Km.7 via al Magdalena Campus la Nubia, Manizales.
Corresponding author: Carrera 4 # 22-61, Módulo 6, Oficina 303, Bogotá, D.C, Colombia. Tel.: +57 (1) 242 7030x1440–1444; fax: +57 (1) 282 6197. Email: [email protected]
Abstract This research investigated and evaluated a biorefinery for processing avocado Hass variety into microencapsulated phenolic compounds extract, ethanol, oil and xylitol. Avocado was first characterized for its potential valuable compounds; then, the techno-economic and environmental aspects of the biorefinery were developed and finally the total production costs and potential environmental impact of the proposed biorefinery were investigated. Four scenarios of the biorefinery were evaluated with different extent of mass and energy integration as well as the incorporation of a cogeneration system. Results indicated that the main fatty acid in the pulp of the investigated avocado variety was oleic acid (50.96%) and that this fruit contained significant amount of holocellulose (52.88% and 54.36% in the peel and seed, respectively). Techno-economic and environmental assessment suggested an attractive opportunity for a biorefinery for complete utilization of the avocado fruit as well the importance of the level of integration.
Keywords: Biorefinery, Avocado, ethanol, xylitol, phenolic compounds extracts, oil, techno-economic analysis, environmental analysis.
Colombia enjoys 433 species of native fruits that represent one of the most diverse varieties of fruits in the world (Restrepo et al., 2014). Colombia leads in biodiversity per square kilometer and has the second largest number of plant species in the world, 51,220, some of which are exotic fruits containing important and valuable compounds (Restrepo et al., 2014). Avocado (Persea Americana mill.) is one of the most important fruits that is cultivated in Colombia and in 2013 Colombia produced 303,340 tons of avocado, accounting for 6.43% of the global avocado production (FAOSTAT, 2017). Colombia is the third largest avocado producer in the world after Mexico (1,467,837 tons) and Dominican Republic (387,546 tons). Avocado varieties that are cultivated in Colombia include: Choquette, Santana, Lorena, Semil, Booth-8, Fuerte and Hass. The Hass variety has the highest among-varieties oil content and accounts for 38% of the total Colombian avocado production (ASOHOFRUCOL, 2013). Typically, the avocado fruit is either consumed fresh or its pulp is processed into fresh, frozen and shelf-stable spreads, such as guacamole, fresh chilled halves and frozen cubes (Barbosa-Martin et al., 2016). In addition to oil, the avocado pulp contains different bioactive phytochemicals such as: carotenoids, vitamins B, C and E, D-mannoheptulose, βsitosterol, persenone A and B, minerals (potassium, phosphorous, calcium, iron, sodium) and essential amino acids such as valine, lysine, phenylalanine, isoleucine, threonine and methionine (Barbosa-Martin et al., 2016; López-Cobo et al., 2016).
The avocado oil can be extracted industrially at a yield of 21% - 30% of the fruit weight. The avocado oil contains, high levels of mono-unsaturated fatty acids (around 70% of oleic acid) and other valuable compounds with health benefits such as tocopherols, phytosterols, lutein and vitamins (López-Cobo et al., 2016). The seed and peel of avocado are obtained as residues after its processing. The avocado seed (15% - 16% of the fruit weight) is one of the best sources of dietary fiber and it contains valuable compounds such as fatty acids, polyphenols, steroids, antioxidants and potassium (Barbosa-Martin et al., 2016). Avocado seed also contains saponin, flavonoids, phenols and cyanogenic glycosides (Arukwe et al., 2012) phytosterols, triterpenes, furanoic acids, flavonol dimers and proanthocyanidins. Avocado peel has been reported to contain flavonoids, saponins, tannins, phenols and steroids and thus exhibits antioxidative activity (Arukwe et al., 2012; Saavedra et al., 2017). The avocado processing industry generates significant amounts of waste (peel and seed) that can thus potentially be processed to yield value added products for which promising and significant applications in the food and related industries exists. The latter, along with the valuable oil that can be extracted from the fruit’s pulp present tangible attractive opportunities to enhance the agroindustry of the avocado fruit. It has to be noted that the functional and commercial value of some of the compounds that can be separated and extracted from all parts of the avocado fruit can be enhanced by application of advanced technological approaches such as microencapsulation, nanofiltration, cryogenic distillation, etc. This approach can be carry out using the biorefinery processing concept and configuration that integrates biomass conversion processes in a given scheme for the production of an array of different products. Among such products are: biomolecules, biomaterials, bioenergy and biofuels, etc. (Davila et al., 2017; Moncada et al., 2014a, 2013). The objective of the presented research was to design and assess a biorefinery for processing avocado into phenolic compounds extract, ethanol, xylitol and oil. Four scenarios of the biorefinery, with different extent of mass and energy integration as well as the incorporation of a cogeneration system, were investigated for their techno-economic and environmental aspects.
Materials and methods
The methodology employed in this work consists of two parts; the first one describes the determination of holocellulose content in avocado peel and seed as well as the fatty acid profile of avocado’s pulp. The second part describes the process simulation that was used in order to obtain the mass and energy balances of the biorefinery, utilizing Aspen Plus V8.0 (AspenTech: Cambridge, MA). In the first step of simulation,
the physicochemical properties of all the compounds involved in the simulation were obtained from the National Institute of Standards of Technology (NIST, 2013). In order, the Non Random Two Liquids (NRTL) and Unifac models were used in the liquid phase. In all cases, the activity coefficients of all the compounds were estimated from the compounds’ mole fractions in the liquid phase. The Hayden O’Conell (HOC) equation was used to calculate the properties of the relevant compounds in vapor phases (Jaramillo et al., 2012). Thus, these methods allow successful calculation of phase equilibria in mixtures with nonconventional compounds (Jaramillo et al., 2012). The second step of the simulation consisted of economic analysis and was carried out using Aspen Process Economic Analyzer (AspenTech: Cambridge, MA, USA). The third step consisted of environmental analysis, which was carried out using WAste Reduction Algorithm (WAR) developed by the US Environmental Protection Agency (Young and Cabezas, 1999).
2.1 Experimental part 2.1.1 Raw material Fully ripened Hass avocado was purchased at a local market in Manizales Colombia. The fruit was thoroughly washed with distilled water then, the pulp, peel and seed of avocado were manually separated. The pulp was immediately used for oil extraction and analysis of fatty acid profile while the peel and seed were each milled to obtain particles that pass mesh 20 (850 μm) sieve and be retained on mesh 80 (180 μm) sieve (Hames et al., 2014). Avocado fractions (peel, seed and pulp) that had been obtained and treated in this way were then stored at -20 ºC pending analysis.
2.1.2 Composition analyses Moisture content was determined, in triplicates, according to the Technical Report NREL/TP 510-42620 (Hames et al., 2014). In short, weighed samples were placed in aluminum weighing dishes and were then dried at 105 ºC to a constant weight. The moisture content was calculated according to equation (1). (1) Water and ethanol extractive contents were determined, in triplicates, according to the procedure described for lignocellulosic biomass (Han and Rowell, 1997). It has been established that water extractives contained organic material, sugars and nitrogenous materials, minerals, etc., while the ethanol extractives consisted of chlorophyll, natural pigments, etc. (Han and Rowell, 1997).
Briefly, extraction with 250 ml of either solvent (water or ethanol) was carried out using a Soxhlet extraction unit for 24 hours at the corresponding boiling temperature of the solvent. The difference in mass prior and following extraction was used to calculate the extractive content according to equation (2). (2) Holocellulose was determined, in triplicates, according to ASTM Standard D-1104 (Han and Rowell, 1997). Briefly, the method is based on treating the plant-derived material with a mixture of water and acetic acid at 70 °C for 60 minutes. The latter allows complete separation of the fiber from the lignin components. The mass difference prior and following the acid treatment was used to calculate the holocellulose content according to equation (3). (3) The cellulose is defined as the insoluble residual part obtained after treating the biomass with NaOH (Han and Rowell, 1997). Briefly, an amount of 1 g of extractives-free holocellulose sample was treated with 10 ml of NaOH solution at 17.5% (each 5 minutes until complete 45 minutes) at 20 ºC. Then, the mixture was filtered at 350 mmbar and the separated solid phase was washed, first with 100 ml of a NaOH solution at 8.3% and then with 100 ml of distilled water. Then, the samples were dried at 40 ºC. Finally, the samples were weighted and the cellulose content was calculated according to equation (4) and the hemicellulose content was determined from difference between holocellulose and cellulose content measured. (4)
Lignin content of the samples was determined according to TAPPI T-222 (Han and Rowell, 1997). Briefly, 200 mg of sample were placed into a 100 ml glass centrifuge tube. Then, 1 ml of H 2SO4 (72% w/w) was added to each 100 mg of sample. Then, the tubes were incubated in a water bath at 30 ºC for 60 minutes. After, the sample was treated in an autoclave at 121°C, 15 psi, for 60 min. Then, samples were removed from the autoclave and the lignin was filtered off, through glass fiber filters under reduced pressure (350 mmbar), keeping the solution at 70 ºC. Residues were washed with hot water (70ºC) and dried at 105 °C overnight in an oven. Finally, lignin content was calculated according to equation (5).
Ash content was determined according to the Technical Report NREL/TP 510-42622 (Sluiter et al., 2008). Triplicate samples were incinerated at 575 ºC for 3 hours and the ash content was calculated according to equation (6). (6)
2.1.3 Oil extraction A thermo-mechanical extraction procedure (Wong et al., 2010) was used. The pulp was macerated to a paste, using a porcelain maceration unit; then the paste was placed in a beaker and mixed at 250 rpm for 60 minutes at 50 °C (Schott Duran, Germany), using a mechanical stirrer (Model AX686/1 AUXILAB S.L.). It has been reported that elevated temperature promotes the extraction of the oil from the oil-containing cells without affecting the quality of the oil (Wong et al., 2010). The oil phase was separated from the pulp in two steps of centrifugations. First, the mixture was spun in a high-speed decanting centrifuge (Hermle Z 206 A, Germany) at 6,000 rpm for 10 minutes at 20 ºC and the separated oil was collected with a syringe. This oil was then spun at 12,000 rpm for 10 minutes at 20 ºC using a high-speed decanting centrifuge (Hermle Z 32 HK, Germany). Following centrifugation, the oil phase (supernatant) was collected using a syringe. Finally, oil was stored at -20 ºC pending for fatty acid profile analysis.
2.1.4 Fatty acid profile The fatty acids profile of the extracted oil was determined using the procedures for determination of fatty acid and isomers composition based on the regulations “Regl2568/91 and “Regl1429/92”, respectively. Methyl esters were prepared by derivatization of oil (AOAC, 2000), 1 ml of boron trifluoride-methanol solution (14%, Sigma Aldrich, St Louis, USA) was added to 50 μl of oil, and the mixture was mixed for 15 seconds and was then placed in a water bath (Thermo Fisher Scientific model 2870, MA USA) at 80 ºC for 40 minutes. Then, 2 ml of hexane (98%, Sigma Aldrich, St Louis, USA) were added and the mixture was mixed for 30 seconds. The treated mixture was allowed to separate into 2 phases over a period of 2 hours at room temperature then, the supernatant (containing methyl esters), was collected for fatty acid profile determination. For determination of the fatty acid composition, 1 μl of the sample was injected a Gas Chromatography (Hewlet Packard 5890 A, Minnesota USA) with a flame-ionization detector. A total retention time of 40 min on a fused silica column (50 m of length x 0.25 mm ID, Sugerlabor) was used. Helium, at a flow rate of 1 ml/min served as carrier gas. The temperature of the injector, and oven were set at 250 ºC and 210 ºC, respectively. The fatty acids composition of the avocado oil was determined according to Regl2568/91.
2.2 Process simulation Figure 1 depicts the proposed scheme of a biorefinery for avocado processing into ethanol, xylitol, microencapsulated extract of phenolic compounds and oil. Additionally, a cogeneration plant was evaluated for supplying part of the energy requirements of the biorefinery. The inlet flow of raw material was set at 10,000 kg/h and the biorefinery was planned to operate 8,000 hours, annually. The constituent plants of the biorefinery have been designed to produce microencapsulated extract of phenolic compounds (extraction and microencapsulation plant), xylitol (xylitol plant), ethanol (ethanol plant) and oil (oil extraction plant). A plant for sugars (xylose and glucose) production was required for generating the feedstock for the xylitol and ethanol plants. The process description and considerations that have been made in designing each one of the constituent plants of the biorefinery are discussed below.
Figure 1. Scheme of the biorefinery based on avocado
2.2.1 Extraction, microencapsulation and sugar plants The extraction of phenolic compounds is carried out using an enhanced-fluidity liquid extraction process (EFLE) with CO2 and ethanol (Ceron et al., 2012). Peel and seed of avocado are milled to a particle sizes smaller than 0.45 millimeters. This size reduction results in a large surface area of the treated material that enhances the extraction yield while improving the ease of handling the solid material (Dávila et al., 2014). Then, the raw material is dried to 7% moisture, while limiting deterioration of antioxidative compounds, at 40 ºC (Ceron et al., 2012). The resulted mass of dried particles is then placed in the extraction vessel where it is mixed with CO2 (solvent) and ethanol (co-solvent). The extraction has been designed to operate at supercritical conditions of 300 bars and 40 ºC. To reach these supercritical conditions, the CO2 is liquefied, using a heat exchanger, and then pressurized to 300 bar, by a pump. The treated CO 2 is transferred to the extraction chamber into which ethanol (co-solvent) was pumped at 300 bar. After extracting of the antioxidant compounds from the solid matrix for 120 min, the extract is transferred to a collecting vessel operated at 50 bar and 25 ºC where the CO 2 is separated from the solid phase and the liquid extract. The process has been designed to allow recovering about 90% of CO2 that is recycled back to the process, thus only about 10% of new solvent has to be added to the process. The extract is then separated from the solid material and then it is concentrated by evaporation. This allows separating part of
the co-solvent (ethanol) and recycling it back to the process. However, because part of the ethanol is lost in the depressurization, there is a need to add new ethanol to the process. Finally, the extract is microencapsulated by spray drying, using maltodextrin as wall material or carrier agent (Warner, 2014). After being filtered to remove particulate solid material that can clog the nozzle of the spray dryer the extract is mixed with a maltodrextrine DE10 solution 54% (w/w) at a carrier-to-extract ratio of 2:1 (Pang et al., 2014). The mixture is spray dried using air at 0.06 MPa and 125 ºC and the microcapsules are recovered in a cyclone and collected. Figure 2(a) depicts the process flow diagram for the extraction and microencapsulation plant. For sugar plant, the solid material (Spent biomass rich in holocellulose) that is obtained from extraction plant is used for sugar production according to the scheme depicted in Figure 2(b). In order to improve the cellulose accessibility, hydrolysis, using diluted sulfuric acid (2% v/v) at 121 ºC for 90 minutes is carrying out (Jin et al., 2011). Following the acid hydrolysis, the liquid stream is separated from remaining solids and is neutralized to a pH of 6.5, using 2% (w/v) NaOH solution. The precipitate (Na 2SO4) is separated by centrifugation (Mussatto et al., 2013) and the pH-adjusted hydrolysate (rich in xylose) is then directed to the xylose plant. The solid fraction (rich in cellulose and lignin) is hydrolyzed enzymatically at 50 ºC using endo-β-1,4 gluconases, according to a kinetic model that describes the decomposition of cellulose to cellobiose and glucose as well as hydrolysis of cellobiose to yield glucose (Morales-Rodriguez et al., 2011). Upon completion of the enzymatic hydrolysis, the liquid phase (rich in glucose) is separated from the remaining solids (rich in lignin) by filtration.
Figure 2. Schemes for extraction and microencapsulation plant (a) and sugar plant (b)
2.2.2 Xylitol and ethanol plants The hydrolysate (rich in xylose) from sugar plant is used to produce xylitol. The hydrolysate is concentrated to 70 g/l by a flash evaporator at 121 ºC and 1 bar and is then cooled to 30 ºC. The concentrated hydrolysate is fermented, using Candida guilliermondii at 30 ºC and stirring rate of 200 rpm, according to the optimized conditions that have been reported by Mussatto et al. (Mussatto & Roberto 2008). The fermentation generates CO2 that is separated from the liquid stream that is then filtered to separate the cell biomass. Finally, the xylitol stream is concentrated in a flash evaporator at 40 ºC and the xylitol is crystallized out using ethanol (95.3% w/w) (Vyglazov, 2004). Figure 3(a) depicts de process flow diagram for the xylitol plant.
Ethanol is produced from the glucose fraction that has been generated by the sugar plant according to the process diagram that is depicted in Figure 3(b). The glucose is fermented to ethanol using Saccharomyces cerevisiae, at 37 ºC, according to the kinetic model that has been proposed by Rivera et al. (Rivera et al., 2006). Following the fermentation, the ethanol concentration is increased to 96%, using distillation and rectification towers. The ethanol is then dehydrated by molecular sieves according to process conditions and yield that have been reported by Quintero et al. (Quintero et al., 2007).
Figure 3. Schemes for xylitol plant (a) and ethanol plant (b)
2.2.3 Oil extraction and cogeneration plants A thermo-mechanical method for extracting oil from avocado pulp was used. This method consists of a moderate temperature (50 ºC) at atmospheric pressure grounding the pulp to a paste by means of mechanical agitation (60 rpm) from 40 to 60 minutes (Wong et al., 2010). This procedure has been shown to promote the extraction of the oil from the oil-containing cells without adversely affecting the quality of the oil (Wong et al., 2010). The oil (supernatant phase) is then separated and centrifuged at 6,000 rpm and the recovered oil is cooled to 25 ºC. Figure 4(a) depicts the oil extraction plant. An electricity generation scheme was proposed to take advantage of the remaining solid stream (rich in lignin) from sugar plant. For this purpose, a biomass-integrated gasifier/gas turbine (BIG/GT) scheme is used for achieving an electricity production at low capital cost. No post-combustion was considered because the relatively high cost associated to capital investments when it is applied to simple-cycle gas turbines (Metz et al., 2005). Thus, an isothermal gasifier is used as well a simple gas turbine to generate electricity. The heat demand was calculated from the energy balance from simulation. To conditioning the solid stream, the feedstock is dried to 10% moisture content at 87 ºC and 1 bar; the dry feedstock is then gasified at 850 ºC at an air-to-fuel ratio of 0.4 (Ruiz et al., 2013). Depending on the composition of the biomass, a mixture of gases consisting of CO, CO2, H2O, H2, and CH4, among other constituents, can be obtained. After gasification, the hot gases are utilized by a gas turbine to produce electricity while the exhausted gases can be used for steam generation (at low pressure) for thermal energy purposes at small industrial scale. Figure 4(b) depicts the process flow diagram for the cogeneration plant.
Figure 4. Schemes for oil extraction plant (a) and cogeneration plant (b)
The objectives, assumptions, conditions and methods used for the principal constituent units in the simulation are summarized in Table 1. Some of the relevant assumptions associated to process simulation, are related to yields at laboratory level taken from literature (It includes yields for units of supercritical extraction, fermentations as well as acid and enzymatic hydrolysis) for obtaining an approximation on the performance of these units at industrial scale. Similarly, for simulation purposes, the feedstock for the biorefinery was assumed as peel, seed and pulp separately thus, not special machines or manual processes were taken into account for separating avocado parts. Finally, the microorganisms for xylitol and ethanol plants are assumed to have not cost taken into account that low concentrations of them are required and that also, are recycled to fermentations processes.
Table 1. Purpose, conditions and methods used for the principal units in the simulation of the biorefinery.
2.3 Economic analysis The economic analysis calculated the production cost per kg of products. The total production cost considered the total raw material (fresh avocado fruit), inputs (reagents, solvents, etc.), utilities, depreciation expense calculated according to equation (7) and operating cost, this last is composed by labor and maintenance costs as well as the operating charges, plant overhead and general and administrative costs. Additionally, tax and interest rates were taken according to the laws in Colombia as 25% and 17%, respectively. The useful life of the project was 10 years and the salvage value for depreciation expense was 20% of the initial capital cost using the straight-line method. Fix capital cost was estimated based on the purchased equipment costs and other direct (installation, instrumentation and control, piping, electrical systems, buildings, yard improvements and service facilities) and indirect (engineering and supervision, construction expenses, legal expenses and contingency) costs. Table 2 shows the economic parameters taken for economic analysis. To obtain a best approach on the simulation and economic analysis, prices and economic data used in this research corresponds to Colombian conditions, raw materials price were estimated according to the purchasing price of local companies as well as transportations costs related to the collection of these raw materials. The economic analysis also included products escalation (5% per year), raw material escalation (3.5% per year), operating and maintenance labor escalation (3% per year) and utilities escalation (3% per year) as an uncertainty of prices through the time. (7)
Table 2. Costs of raw materials and services to the Biorefinery
Calculated for transportation of SBP over a distance of 140 Km with a truck of three axles.
Typical price in Colombia.
Taken from ICIS Prices (ICIS, 2013).
Prices based on Alibaba International Prices (Alibaba, 2013).
Estimated cost of Gas for the years 2015 – 2035 (NME, 2013).
National price in Colombia (Fedebiocombustibles, 2013).
Taken from international prices.
Taken from (Alibaba, 2013).
Taken from (Warner, 2014).
Taken from (Mussatto et al., 2013).
Taken from typical prices in Colombia.
Taken from (Peña et al., 2012).
The economic analysis was carried out considering four scenarios that included several levels of heat and mass integrations, which are described in table 3. Mass integration refers to CO2 and ethanol recovering from extraction plant (Figure 2a) that allows calculating the savings of these inputs. Heat integration was carried out using the Pinch methodology supplied by Aspen Energy Analyzer software and, based on composite curves; the energy saving for all scenarios was calculated.
Table 3. Description of scenarios
Because the total production cost was calculated for each product (xylitol, ethanol and phenolic compounds), the cost associated to other plants was distributed among products plants. Thus, the costs associated with the sugar plant were distributed according to holocellulose (hemicellulose and cellulose) content thereby, 40% (based on hemicellulose content for producing xylitol) and 60% (based on cellulose content for producing ethanol) of the cost associated to sugar plant were assigned for xylitol and ethanol plants, respectively. Similarly, the costs associated with cogeneration plant were distributed according to the energy requirements for each product plant thus, 39%, 58% and 3% for xylitol, ethanol and phenolic
compounds plants, respectively (Davila et al., 2017). The cost distribution of the depreciation expense was assigned according to the units associated to each product plant thus, 40%, 40% and 20% for xylitol, ethanol and phenolic compounds plants were used, respectively. The cost associated to raw materials and inputs were distributed according to the consumption of each plant therefore, 30% of raw material costs was assigned to xylitol plant (Approximately 25% of hemicellulose of the raw material, H 2SO4 for acid hydrolysis, NaOH for neutralization of xylose, C. guilliermondii for fermentation and ethanol for washing xylitol), 65% of raw material costs was assigned to ethanol plant (Approximately 27% of cellulose, enzyme for enzymatic hydrolysis and S. cerevisiae for fermentation) and 5% of the raw material costs was assigned to phenolic compounds plant (CO2, ethanol and maltodextrine). Finally, utilities cost was distributed according to the energy consumption of each product plant thus, 39%, 58% and 3% for xylitol, ethanol and phenolic compounds plants, respectively (Davila et al., 2017).
2.4 Environmental analysis The environmental analysis evaluated eight environmental impact categories which are: Human Toxicity Potential by Ingestion (HTPI), Human Toxicity Potential by Dermal and Inhalation Exposure (HTPE), Terrestrial Toxicity Potential (TTP), Aquatic Toxicity Potential (ATP), Global Warming Potential (GWP), Ozone Depletion Potential (ODP), Photochemical Oxidation Potential (PCOP) and Acidification Potential (AP). The Potential Environmental Impact (PEI) of the process was calculated per kilogram of products (Dávila et al., 2014). The PEI was calculated according to equation (8). Where the sum over j is taken over the streams of outputs (out), the sum over k is taken over all chemicals k, Ṁj(out) is the mass flow rate of the outlet stream j, Xk,j is the mass fraction of chemical k in the outlet stream j and Ψj corresponds to the PEI of the chemical associated to an impact category, which are supplied by the software (Young and Cabezas, 1999).
Results and discussion
3.1 Chemical characterization The results of the compositional characterization of avocado peel and seed are presented in Table 4. The moisture content of both, peel and seed was similar to what has been previously reported. For instance, a moisture content of 7.66% has been reported for seed of Hass avocado variety (Barbosa-Martin et al., 2016). Moisture content of 5.33% and 9.22% for peel and seed of avocado, respectively, has been previously reported (Arukwe et al., 2012). The mass of total extractives was 34.38% and 35.95% (on dry basis) of the extracted peel and seed, respectively. Other studies have reported that water and ethanol extractives content of avocado ranged from 15.33% to 23.11% (Indumathi and Krishna, 2014). It has been reported that extracts from avocado seed and peel (obtained by soxhlet method) contain flavonoids, steroids, terpenoids, saponins and tanins as well as phenolic compounds, alkaloids and steroids. (Arukwe et al., 2012; Warner, 2014). Other valuable compounds found in peel and seed of avocado are perseitol, quinic acid, chlorogenic acid, quercetin derivates, rutin citric acid, tyrosol glucoside, penstemide and vanillic acid glucoside (López-Cobo et al., 2016).
Table 4. Chemical characterization for peel and seed of avocado
Ash content of avocado peel (1.04%) and seed (0.87%) was similar to what has been previously reported: 1.50% and 1.29% for peel and seed, respectively (Vinha et al., 2013). Holocellulose content for peel (52.88%) and seed (54.37%) was similar to what has been reported for avocado peel (45.74%) and seed (51.21%), respectively (Arukwe et al., 2012). Other researchers have reported that holocellulose content of avocado seed was 27.45% while cellulose content of avocado peel ranged from 18.7% to 28%, depending on the repining stage (Barbosa-Martin et al., 2016). Both, peel and seed of avocado contain significant amounts of holocellulose and thus can be used as feedstock for obtaining C 5 and C6 sugars (Dávila et al., 2014) that, in turn, can then be processed to yield other valuables products such as biofuels, organic acids, bioenergy and biomaterials. Results of this research along with those reported earlier thus clearly suggest that avocado waste streams (peel and seed) can be transformed into valuable products for different food applications (Saavedra et al., 2017).
3.2 Fatty acids profile Results of fatty acid constituents indicated that the presence of fatty acids ranging from C14 to C18. The major fatty acids of the avocado oil were oleic acid (C18:1, 50.96%), palmitic acid (C16:0, 24.74%,), linoleic acid (C18:2, 15.07%), palmitoleic acid (C16:1, 5.97%) and linolenic acid (C16:3, 1.66%). Other fatty acids such as myristic (C14:0, 0.05%), margaric (C17:0, 0.03%), margaroleic (C17:1, 0.08%), estearic (C18:0, 0.84%), arachidic (C20:0, 0.14%), gadoleic (C20:1, 0.27%), behenic (C22:0, 0.27%), lignoceric (C24:0, 0.06%) and other isomers trans of C18 were present only at low concentration. In agreement with what has been previously reported for avocado oil, oleic acid was the main fatty acid in the Hass avocado oil and constituted more than 50% of the total fatty acids content (Pedreschi et al., 2016). Although a few variations of fatty acid composition of avocado oil have been reported by several authors, it is important to note that postharvest ripening strategies did not have any detrimental effect on the fatty acid profile while other variables such as ripening stage, region, seasonal variations and processing methods had a significant effect on the fatty acid composition of avocado oil (Pedreschi et al., 2016). The selection and application of an appropriate extraction method, as an integral part of the processing of avocado in a biorefinery is of great importance. The latter has to be considered based on the combined influence of technological aspects, compositional and oxidative stability aspects as well as economic considerations.
3.3 Techno-economic analysis The mass balance for the avocado biorefinery reveals that the ethanol production corresponds to 47.4 kg (60.08 litters) per ton of solid material (seed and peel) that leaves the extraction and microencapsulation plant. It has been reported that 31.8 kg of ethanol could be produced per ton of avocado seeds (Woldu and Tsigie, 2015). Other type of biomass, olive tree pruning has been processed according to the biorefinery concept to yield 63.37 litters of ethanol per ton of feedstock (Hernández et al., 2014). Results indicated that xylitol was obtained at a yield level of 25.51 kg per ton of solid material (peel and seed). This value is lower than what has been reported by others for lignocellulosic biomass. For instance, 103 kg of xylitol per ton of lignocellulosic feedstock consisting of brewer’s spent grains has been reported (Mussatto et al., 2013). Results indicated that oil extraction yielded 142 kg of oil per ton of avocado. Other authors reported an extraction yield of 100 kg of oil per ton of avocado at a pilot plant scale level (Martínez Nieto et al., 1992). Techniques such as cold-press oil extraction and supercritical CO2 extraction allowed obtaining 15.8% (v/w) and 39.8% (w/w) of avocado oil, respectively (Costagli and Betti, 2015). These techniques are more expensive than traditional methods for oil extraction and therefore a thermo-mechanical extraction process
followed by centrifugation seems to be the method of choice for extraction of avocado oil in developing countries (Bizimana et al., 1993), such as the country in which this research was conducted. Table 5 presents the cost and cost share of each of the evaluated scenarios of the avocado biorefinery. For scenario 1, results indicated that the most important economic factor of the total production cost was utilities, reflecting the high-energy consumption of a process configuration that does not include heat integration. However, results indicated that when mass and energy integration (scenarios 2 and 3) as well as a co-generation system (scenario 4) have been introduced, the raw material became the most important economic factor in affecting the total production cost, reflecting the relatively high cost of avocado (fresh fruit). In agreement with previous reports, results of the study highlighted the merits of integration strategies as useful approaches for designing multi-product biorefineries (Davila et al., 2017; Moncada et al., 2013). Results indicated that the level of mass and energy integration in the avocado based biorefinery is a major factor that should be considered when reducing the total production cost of all the products that are associated with a given biorefinery concept is considered.
Table 5. Cost and share of avocado biorefinery
When utilities consumption is considered, results indicated that heat integration is of importance and, as demonstrated in scenario 2, the cost of energy consumption of the entire biorefinery could be reduced by 69.48%, relative to scenario 1 (without heat integration). This result is in agreement with earlier reports about heat integration in a biorefiney. For example, a reduction of 63.23% and 54.91% of the total energy requirements of biorefineries for processing fruits and brewer’s spent grains, respectively has been reported (Davila et al., 2017; Mussatto et al., 2013). Including a cogeneration system in the avocado biorefinery (scenario 4) is unattractive because the reduction of energy consumption is only marginal (0.63%) in comparison to what can be accomplished with heat integration (scenarios 2 and 3), however, it should be carried out in order to attain the highest possible reduction in energy requirements. It has to be noted that in addition to reducing utilities consumption, heat integration can lower the impact of the biorefinery on the environment, due to the lower consumption of external fuel that affects the green house gases (GHG) emissions (Moncada et al., 2013). Results indicated that the cost of raw material (fresh avocado fruit) gained importance as a major economic factor when the level of integration increased, reflecting the lower cost share of utilities and thus, higher cost share for raw material. Additionally, the relatively high price of avocado fruit (0.47 USD/kg, table 2) renders this raw material an important economic factor in the biorefinery design. The latter is in contrast to cost of raw materials in biorefineries that are aimed at processing second generation raw materials that are cheaper; for example, 0.042 and 0.021 USD/kg of brewer’s spent grains and blackberry residues,
respectively have been reported (Dávila et al., 2017; Mussatto et al., 2013). The high costs associated with cost of feedstock and inputs as well as the high energy consumption have been noted by others as the most important economic factors that affect significantly the design of integrated biorefineries (Moncada et al., 2014b). Thus, the highest levels of energy and mass integrations should be practiced in order to ensure the most profitable scenario of a biorefinery, in particular when biorefineries that use first generations feedstocks such as avocado are considered. Table 6 depicts the cost and cost share of each input over the total input costs for scenarios without (scenarios 1 and 2) and with (scenarios 3 and 4) mass integration. Studying the influence of inputs on the total production cost indicated that, enzyme is the most expensive input that contributes to more than 50% of the total cost of inputs in all scenarios (See table 6). This fact has also been highlighted by other authors, who suggested that despite the significant progress in developing robust enzymes, enzymatic processes included in the biorefinery concept are still not economically competitive (Long et al., 2016). Therefore, research directed at the selection of appropriate enzyme preparations should be addressed in the biorefineries design to enhance the use of inputs that can lower mass consumption and thus, lower cost of inputs in the biorefinery.
Table 6. Cost and cost share of inputs for avocado biorefinery
Ethanol is the second input with high contribution to total cost of inputs, however, after mass integration there is a possibility for saving 57.65% ethanol. Similarly, a saving of 23.70% water was achieved after mass integration. This extent of savings for ethanol and water indicates that mass integration is a very useful and necessary approach for reducing inputs consumption, environmental impact as well as total production cost. It has been reported that, residual water from a biorefinery should be recovered at the highest level possible (as in this research) in order to address the large volume of water that is needed in biotechnological processes and presents a major concern in the processes design (Mussatto et al., 2013). When process streams are integrated, such as when lignocellulosic biomass (peel and seed of avocado) is used for producing ethanol and xylitol, production cost can decrease significantly. In fact, one of the greatest challenges in the production of second-generation biofuels is the cost associated with transportation and availability of raw materials (Moncada et al., 2013). Therefore, integrating processes into one biorefinery (such as in this research) is necessary for reaching the most profitable biorefinery configuration. Results of this research also suggest that integration of first and second generation biorefineries should be considered as an integrated way for processing a given raw material (such as avocado) into value added products in a feasible way, in agreement with suggestions of others (Moncada et al., 2015).
Figure 5 depicts the sale-price-to-total production cost ratio (Mussatto et al., 2013) which was calculated using the prices reported in table 2. For the best scenario (scenario 4), ethanol, xylitol and oil exhibited the highest production cost (3.71, 32.77 and 212 USD/kg respectively) in comparison to their sale prices (See table 2). Microencapsulated phenolic compounds had the lowest production cost (1.8 USD/kg) and, in light of the high sale price for microencapsulated phenolic compounds (167.48 USD/kg from table 2), it presents the most promising value added product to be obtained from a biorefinery based on avocado. This fact suggests that, under biorefinery concept, a given product, such as the microencapsulated one in this research (or a few products) can provide a “subsidy” for the production of other products (ethanol, xylitol and oil in this research), similar to what has been reported for a biorefinery based on sugarcane (Moncada et al., 2013). Thus, scenario 4 has the highest sale-price-to-total-production cost ratio indicating that mass and energy integrations can reduce significantly the total production cost while scenario 1 (without mass and energy integrations) is not attractive because it increases the total production cost which, in turn, reduces the sale-price-to-total-production cost ratio.
Figure 5. Sale price to total production cost ratio for avocado biorefinery
Despite of the low production volumes of microencapsulated phenolic compounds (6.27 kg/h), the high sale prices can compensate the high productions costs associated to all products in the biorefinery therefore, the sale-price-to-total production cost ratio is higher than unity for all scenarios (see figure 5). Other authors found that the production of anthocyanins under biorefinery concept has the highest contributions on sales (Moncada et al., 2013) thus, the extractions and production of bioactive compounds (such as microencapsulated phenolic compounds in this research) in these kinds of processes should be considered because the importance and relevance to the food, pharmaceutical and chemical industries. In light of the results obtained above, a biorefinery based on avocado could be attractive in the Colombian context for several reasons. For instance, taken into account that avocado as feedstock in this biorefinery corresponds to 26.37% of the entire Colombian production then, a potential impact on food competition can be minimal. Besides, potential improvements on agro-industrial chain of avocado in Colombia could be obtained if valuable products from this fruit are processed such in this research. This point of view allows enhancing the actual added value of avocado, obtaining valuable products that can open regional and foreign markets.
3.4 Environmental analysis Figure 6 depicts the leaving PEI where, the most affected environmental categories correspond to HTPI, TTP, PCOP and AP. The latter reflects the impact of the organic matter that is included in the liquid streams, such as stillage, leaving the biorefinery. Similar observation was reported by (Moncada et al., 2014a) for a biorefinery for sugarcane processing where HTPI and TTP were affected by organic matter content of the effluents. PCOP is associated with CO2 production (Montoya et al., 2006) and thus, the CO2 leaving the xylitol and ethanol plants as well as the CO2 losses from phenolic compounds plant contribute to PCOP. Similar results have been reported by (Hernández et al., 2014) where PCOP was the most important environmental category that contributed to the PEI of a biorefinery for olive stone processing. Acidification potential (AP) exhibited high values that corresponds to the consumption of external energy to cover the energy requirements (Moncada et al., 2014a).
Figure 6. Leaving PEI for avocado biorefinery
Both, heat and mass integrations lowered the PEI but mass integration had the most significant effect due to the savings of ethanol (57.65%) and water (23.70%). Mass integration (scenarios 3 and 4) reduces PEI by 2.65 and 3.37 folds in comparison to biorefineries with heat integration (scenario 2) and without heat integration (scenario 1), respectively. Other authors found reduction of PEI by a factor of 2.9 and 4 for biorefineries considering heat and mass integrations, respectively, relative to a biorefinery without any level of integration (Moncada et al., 2015). Results indicated that scenarios 3 and 4 were the most environmentally friendly, reflecting the lower leaving PEI due to not only to the savings in mass (ethanol and water) but also lower heating and cooling fluids for energy requirements (69.48% with heat integration). Because mass integration had a positive impact from both the environmental and economic points of view, recovering and recycling process streams, at highest possible level, allows a better waste management as well as an efficient use of energy that converges to a better economic performance and lower environmental impact of the biorefinery. Integrated first and second biorefineries also contribute to lower PEI as well decreasing organic loads (peel and seed of avocado) in waste streams, due to the utilization of these second-generation raw materials for manufacturing valuable products (ethanol and xylitol). The environmental performance could be improved if third generation biorefineries (those based on microalgae) are integrated for example, capturing CO2 and using it as substrate for microalgae growth, it should decrease the green house gases and PEI (Moncada et al., 2015).
Conclusions Results indicated that processing avocado according to the biorefinery concept is an attractive opportunity for an integrated processing of the fruit into a series of valuable products, using the pulp, peel and the seed of the fruit. Integration of first and second-generation biorefineries should be addressed in order to attain process profitability and that mass and energy integration have to be included in order to reduce significantly the total production cost and the potential environmental impact. The ratio of sale-price-tototal-production-cost suggests that all of the evaluated products have potential for further applications, from the economic and environmental points of view.
Acknowledgements The authors express their gratitude to Junta de Andalucía (Proyecto de Excelencia AGR-6103) as well as to Asociación Universitaria de Postgrado (AUIP) and “Postgraduate Outstanding Student Scholarship” of the Universidad Nacional de Colombia by the financial support in the travel and other expenses for carrying out this research.
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Table 1. Purpose, conditions and methods used for the principal units in the simulation of the biorefinery. Conditions and unit Unit
specifications PRETREATMENT PLANT Drying to 5.12% moisture Dryer
40 ºC, 1 bar NRTL * No Atmospheric dryer (Model in Aspen Plus: Shortcut Dryer) Size reduction to 0.45 mm 1 bar N.A. N.A. Mill (Model in Aspen Plus: Jaw Mill) PLANT FOR EXTRACTION AND MICROENCAPSULATION OF PHENOLIC COMPOUNDS Extract phenolic 40 ºC and 300 bar and 54% User model (Cerón at al., 2012) Extractor compounds (w/w) maltodrextrine DE10 (Yield of solution 59.3%) (Model in Aspen Plus: Sep) Recovering part of the 40 ºC, 0.3 bar NRTL-HOC N.A. Evaporator ethanol (Concentration of Standard tube vertical the extract) evaporator, one effect (Model in Aspen Plus: Flash2) Removing particles from 55 ºC, 1 bar NRTL-HOC N.A. Filter the extract (Model in Aspen Plus: Rotary Vacuum filter) Microcapsules formation 125 ºC, carrier-to-extract ratio of User model (Pang et al., 2014, Spray drying 2:1. (Yields of Tatar et al., 2015) (Model in Aspen Plus: Shortcut 82.08%) Dryer) SUGAR PLANT Enhancing the efficiency 121 ºC, 1 bar, (2% v/v of User model (Jin et al., 2011) Acid hydrolysis of recovering cellulose H2SO4) (Yield of 0.6 and xylose g/g) Neutralization of acid 25 ºC, 1 bar, pH of 6.5 with 2% User model (Mussatto et al., Neutralization w/v of NaOH NRTL-HOC 2013) (Model in Aspen Plus: RYield) Glucose production 50 ºC, 1 bar, 7% (wt) using endo- User model (Morales et al., Enzymatic β-1,4,glucanases (Concentration 2011) hydrolysis Agitated tank of 6.14 g/l) (Model in Aspen Plus: RYield) Reaction temperature 121 ºC, 1 bar NRTL-HOC N.A. Heat Exchanger XYLITOL PLANT Removing part of the 121 ºC, 1 bar (until 70 gr/l) NRTL-HOC N.A. Evaporation water (Concentration of Standard tube vertical xylose) evaporator, one effect (Model in Aspen Plus: Flash2) Production of Xylitol 30 ºC. Candida guilliermondii, User model (Mussatto and Fermentation 200 rpm (Yield of 0.78 Roberto 2008) (Model in Aspen Plus: RYield) g/g) Xylitol crystallization 40 ºC, Ethanol at 95.3% NRTL N.A. Crystallizer (Model in Aspen Plus: Crystallizer) Reaction temperature 121 ºC, 1 bar NRTL-HOC N.A. Heat Exchanger ETHANOL PLANT Ethanol production 37 ºC, using Saccharomyces User model (Rivera et al., 2006) Fermentation cerevisiae (Concentration (Model in Aspen Plus: RYield) of 78 kg/m3) Ethanol separation Distillation: 18 trays, 2.5 reflux NRTL-HOC N.A. Distillation ratio, total condenser columns Rectification: 12 trays, 1.8 reflux ratio, total condenser (Model in Aspen Plus: RadFrac) Reaction temperature 121 ºC, 1 bar NRTL-HOC N.A. Heat Exchanger OIL EXTRACTION PLANT Oil extraction 50 ºC, 1 bar NRTL-HOC N.A. Extractor (Model in Aspen Plus: Assuming 70% of yield Decanter) Oil separation 50 ºC, 1 bar, 6,000 NRTL-HOC N.A. Centrifuge
Reaction temperature Heat Exchanger COGENERATION PLANT Syngas generation Gasifier
rpm (Model in Aspen Plus: CFuge) 121 ºC, 1 bar
859 ºC, 60 bar and a ratio of 0.4 air-to-fuel (Model in Aspen Plus: RYield and RGibbs) 1 bar, 70% efficiency (Model in Aspen Plus: Turbine with isentropic efficiency)
* NRTL corresponds to Non-Random Two Liquids model for activity coefficients calculation
Table 2. Costs of raw materials and services to the Biorefinery Item
Sulfuric acid c
Sodium hydroxide c
Operator labor cost b
Supervisor labor cost b
Electricity cost b
Ethanol at 99.5% f
Avocado fruit k
Avocado oil l
Calculated for transportation of SBP over a distance of 140 Km with a truck of three axles.
Typical price in Colombia.
Taken from ICIS Prices (ICIS, 2013).
Prices based on Alibaba International Prices (Alibaba, 2013).
Estimated cost of Gas for the years 2015 – 2035 (NME, 2013).
National price in Colombia (Fedebiocombustibles, 2013).
Taken from international prices.
Taken from (Alibaba, 2013).
Taken from (Warner, 2014).
Taken from (Mussatto et al., 2013).
Taken from typical prices in Colombia.
Taken from (Peña et al., 2012).
Table 3. Description of scenarios Scenario
Energy integration level
Mass integration level
Scenario 4 *
* Scenario 4 includes heat and mass integrations such scenario 3 but it includes a cogeneration system.
Table 4. Chemical characterization for peel and seed of avocado Component
Avocado (mass %) Peel
7.33 ± 1.15
7.02 ± 0.18
34.38 ± 0.34
35.95 ± 1.95
27.58 ± 1.18
6.48 ± 0.38
25.30 ± 1.24
47.88 ± 2.14
4.37 ± 0.13
1.79 ± 0.04
1.04 ± 0.05
0.87 ± 0.06
Table 5. Cost and share of avocado biorefinery Item
Fix capital investment
Table 6. Cost and cost share of inputs for avocado biorefinery Item
Scenarios 1 and 2
Scenarios 3 and 4
Avocado fruit has significant potential for providing an array of products
Biorefinery based on avocado is attractive for Colombian context
Economic analysis of biorefinery based on avocado reveal reasonable process costs